Fix seriously broken "git pack-refs"

Fix seriously broken "git pack-refs"

Do *NOT* try this on a repository you care about:

	git pack-refs --all --prune
	git pack-refs

because while the first "pack-refs" does the right thing, the second
pack-refs will totally screw you over.

This is because the second one tries to pack only tags; we should
also pack what are already packed -- otherwise we would lose them.

[jc: with an additional test]

Signed-off-by: Junio C Hamano <junkio@cox.net>